The development of quantitative processing has been studied for decades, at least from the time of seminal studies by Piaget on children's reasoning about number and quantity. Beginning with now-classic studies conducted in the 1980s, there has been significant interest in the origins of such reasoning in infancy. Many of these studies are motivated by a desire to understand the core number systems that infants have, and to document infants' precocious abilities to process information about quantity. It is time for a shif in focus, so we can understand the development of these abilities in infancy. The existing literature provides mixed results and few clear answers for why the findings are mixed. It is clear that infants are sensitive to number and quantity. What is less clear is how this sensitivity develops, and how infants focus on some dimensions, such as exact number, and ignore other dimensions, such as total area. Understanding this development is important for several reasons. Effective functioning in the world depends on quantitative reasoning. Even infants need to be sensitive to quantity to make decisions about how to act, track objects, and accurately represent the world around them. In addition, advances in science and technology require strong quantitative abilities, and numerical abilities in infancy and early childhood may be related to quantitative abilities later in childhood. Finally, numerical processing is impaired n a number of developmental disorders, such as William's Syndrome and Fragile X syndrome. A complete understanding of such disorders-and the development of interventions to help children cope with impairments-requires a complete understanding of typical development. Although infants' visual number processing has been studied for decades, there remains little understanding into how this ability develops. Studies have asked whether or not infants attend to number or other features of displays, and what conditions encourage infants' attention to number. These important studies document infants' ability, but have yielded limited understanding into how these processes develop. Research with adults has revealed that processing of small sets of numbers is closely related to other visual attentional and perceptual processes. Thus, it is possible that infants' processing of small sets is also related to those visual cognitive abilities. This project asks how the development of number processing can be understood by relating this processing to other aspects of visual processing. This project will aim to uncover the development of infants' attention to number versus total area (Experiment 1). This is an important question because it will provide understanding into how infants' balance their attention to multiple visual features. Understanding how infants' balance and control their attention in this way has been the topic of research in other domains. The proposed project, therefore, will allow closer ties to those other domains. In addition, this project will examine ho individual differences in number processing are related to individual differences in other cognitive abilities (Experiment 2). Number processing does not happen in isolation. We determine how many items are present while perceiving the items, attending to them, forming memories of them. In addition, some of these other processes may be important for visual number processing (i.e., recognizing how many items are present depends on perceiving the items as individual items). Thus, a complete understanding of number processing requires examining how number processing is related to other visual cognitive processes.